1. Field of the Invention
The present invention relates to a throttle valve opening and closing device for changing a flow rate of intake air introduced to an internal combustion engine for a vehicle.
2. Description of the Related Art
One conventional throttle valve opening and closing device for a vehicle, which has a throttle valve driven by a motor, is disclosed, for example, in JP,A 2-500677 (PCT Laid-Open Publication No. WO88/00404). The disclosed device comprises a return spring for biasing the throttle valve in the closing direction, and a second spring for biasing the throttle valve in the opening direction contrary to the return spring. When the throttle valve is not driven by the motor, the throttle valve is automatically adjusted to an opening degree at which the biasing force of the return spring is balanced by the biasing force of the second spring.
In this specification, an initial position of a throttle valve maintained by biasing means, e.g., a return spring, for biasing the throttle valve to the initial position when the throttle valve is not driven by driving means, e.g., a motor, is referred to as a “default position” in the following description.
The reasons why a mechanism for setting the default position is provided are as follows:                1. Even when no driving forces are generated during parking, the mechanism positively defines a gap between the throttle valve and an intake passage (specifically a wall surface of an intake passage) to prevent sticking of the throttle valve, which is otherwise possibly caused by freezing or contamination occurred between the throttle valve and the intake passage.        2. When the throttle valve opening and closing device is failed and power supply to the motor is stopped, the mechanism is operated to quickly set the position of the throttle valve so as to provide an opening degree at which such a flow-rate of intake air as not stalling the engine is obtained while preventing runaway of the vehicle, or to ensure self-controlled traveling (limp home mode).        
Also, a throttle valve opening and closing device for a vehicle, disclosed in JP,A 11-153053, comprises a link or cam mechanism for transmitting rotation of a motor to a throttle valve, and a return spring for biasing the throttle valve in the closing direction to reduce its opening degree. This device further comprises a mechanism for operating the throttle valve such that, when the motor is rotated in one direction from the default position, the throttle valve is operated once in the closing direction from the default position, and when the motor rotation is further continued thereafter, the rotating direction of the throttle valve is reversed and the throttle valve is operated until reaching a fully opened position.
In the device utilizing the opposed biasing forces to realize the default position as disclosed in the above-cited JP,A 2-500677 (PCT Laid-Open Publication No. WO88/00404), a delay occurs in the operation of the throttle valve when the valve operation is performed across the default position. This is because the direction of a load imposed on the motor for generating the driving force is reversed upon crossing the default position and a control process takes a time until the reversal in the direction of torque is determined. Another problem is that, due to the necessity of providing two springs for developing biasing forces in opposed directions, the mechanism is complicated, thus resulting in deterioration of both mass producibility and mountability to the vehicle.
Also, the device disclosed in the above-cited JP,A 11-153053 is free from the reversal in the direction of spring biasing forces upon crossing the default position, but it requires the steps of, when the throttle Valve must be operated from the default position to the fully opened state, operating the throttle valve once in the closing direction until teaching a fully closed state and then reversing the rotating direction of the throttle valve to operate it from the fully closed state to the fully opened state. Therefore, a delay caused with the reversal in the rotating direction of the throttle valve is not negligible. In other words, looking at a series of valve operations from the default position to the fully opened state, the device disclosed in the above-cited publication has a problem that a useless step of reversing the operation of the throttle valve is included and the throttle valve cannot be operated at a high speed.